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Ampeg SVT 2 Pro bias

Discussion in 'Amps and Cabs [BG]' started by Krügmeister, Jul 22, 2009.

  1. On the back of the SVT 2 there is user adjustments.. it says to set the tubes so that they are green... however when you play a note they flash red while the green lights stay greeen.. is this common.. or am I setting the bias incorrectly?
    If the amp is in standby does that mean that the two lights need to stay solid green and they are biased?

    just found the answer

    Setting Tube Bias
    Allow the unit to warm up at proper AC line voltage for at least 20 minutes. With no input signal present, adjust each
    control so that only the associated green LED is lit. The controls may be slightly interactive. If neither LED is lit, the amp
    is overbiased. This will result in some distortion in the power amp and a generally thin sound. If the green and red LED are
    lit, the amp is underbiased and too much current is flowing in the power tubes. This will give a big, full sound but will also
    reduce the life of the power tubes. Once set, the controls should not have to be changed except as needed for tube
    replacement, or to compensate for tube aging. Note that the AC line voltage may vary from place to place and the LEDs
    will read slightly different. There is no need to fiddle with this every other day. Note that it is normal for the red LEDs to light
    when there is a signal present. Bias 1 Control adjusts the three left (as seen from the rear) power tubes. Bias 2 Control
    adjusts the three right power tubes. By observing the LEds as the Bias Controls are slowly rotated clockwise, a number of
    tube problems can be diagnosed by the user:
  2. edjazzbass


    Sep 13, 2009
    Observation notes: The bias adjustment pots can tell you a lot about the health of your power tube bank.

    Once the amp is energized and in standby and allowed to warm up for a decent period of time like several minutes, with no bass signal plugged into the input, turn the unit HV on by flipping the standby switch to off. the LEDs in the rear of the amp should both be green. It has been my experience that if you have a bass plugged into the amp and you thump the bass or play a note both the green and red leds will flicker with signal input.
    once you stop, the LEDs settle to green. This might change a bit after an hour or so of performance, but generally they sit at two green. both trimmers are closely matched in position too.

    If you adjust the trimmers, the more clockwise you can turn the trimmer before the red LED comes on the better balanced or matched the tubes are. On my SVT II Pro, the trimmer sits at about 45-48%. looking at the screwdriver slot, it appears almost vertical.
    but that setting is different for different tubes. Mine was built in 1998, still uses the original tubes, the 6550s are Svetlanas. I am the original owner. I purchased the unit new in 1998. Never had a problem at all . During use in the last year, I noticed the bias lamps on the #1 side not flickering like usual during a signal input using the bass. Realizing the tubes might be getting old (10 years) I figured this was age and time for a new set... but no.

    I had an issue with the number 1 bias not glowing green. It would however turn red when trimmed past 50% and the amp still would work. At first I thought a tube had gone bad. After swapping the tubes around, I found one of the power tubes to not be glowing . The filament was intermittent. it would come and go. Swapping tubes did not correct the problem. After shutting down the amplifier, I carefully removed the power tubes and removed the power tube bank from the chassis. Since the filament voltage is on pin 2 and 7 of the 6550, I first checked these pins with a multimeter. continuity check everything checked normal. On closer inspection of each pin on the socket was a different story. I found pin 7 on the suspect tube socket to have a cavity worn away from arcing and corrosion around the entire pin. looking closely at the soldered pins on the pc board, each of the remaining pins on that socket only had
    developed tiny cavity rings around the soldered joints. I removed the solder from the tube socket pins and found clumps of flux in the socket pin cavities between the pc board and tube socket pins.
    I cleaned the pc board of the old flux using isopropyl alcohol and a few shots of compressed air. I re soldered the tube socket. I re assembled the board and the SVT II Pro went right back to showing two green LEDs with no problem at all.

    The other five tube sockets looked perfectly silver. only the one (V3) had the evidence of solder flux corrosion that caused the filament pins to eventually separate from the pc board. The 6550s continue to perform great.

    The bias LED not showing Green but showing red indicated a dead tube. Swapping the tubes carefully did not cause the bias lamp indication to shift over to bank 2 . I was able to spot the dark filament in the suspect tube operating the unit in standby with the top cover off.
    The 6550 has 650 volts on the plates when it is in operate mode. If the lid is off the amp... Beware the voltages present on the plates of the 6550 can be lethal. Before I did any tube swapping or removal and repair of the power tube board, I removed the power cord from the amp and allowed it to cool down before i disconnected the filament and HV cables feeding the board.
    If you turn the trimmers fully left, the LEDs will go out, if you turn them to the right the green is supposed to come on and then the red. for my amp the "sweet spot" sits at just about mid point or 45% on the trimmer pot.

    Stay in the pocket,